The present invention relates to an image-retransferable sheet for a dry-processing type image-transferring material which is capable of transferring an image such as letters, signs, figures or the like onto the surface of a desired substance by applying pressure to the image-transferring material whose image-bearing surface is brought into contact with the surface of the substance (hereafter referred to as "pressure-sensitive retransfer"). More specifically, it relates to an image-retransferable sheet of such an image-transferring material produced by thermally printing or transferring an image on the sheet with a thermally transferring type printing device such as a printer, a typewriter, a word processor or the like (hereafter referred to as "heat-sensitive transfer").
As a base sheet (image-retransferable sheets) for dry-processing type image-transferring materials produced by heat-sensitive transfer, JP-A-63-128987 discloses films of polyethylene, polypropylene, fluorine-containing resins, etc., or silicone resin-coated sheets of paper, metal foils, plastic films, etc., which have a smooth surface and exhibit a water-contact angle of at least 95.degree.. (The term "JP-A" used herein means an unexamined published Japanese patent application.)
In formation of an ink image on such a base sheet having a water-contact angle of 95.degree. or more (particularly 105.degree. or more) by heat-sensitive transfer, an ink temperature is necessarily increased to reduce surface tension of the ink and wet the surface of the base sheet to an extent that adhesion of the ink to the base sheet becomes larger than cohesive force of the ink and adhesion of the ink to an ink-donating base film such as a polyethylene terephthalate (PET) film. Thus, high thermal energy is required for the image formation, and it is very disadvantageous to devices for heat-sensitive transfer concerning durability of a thermal head and load to an electric source.
Further, ink images formed on such a surface of poor wettability are easily retransferred merely with little pressure applied thereto because of its poor adhesion to the surface so that, upon retransfer of certain portions of the images, the other portions of the images are undesirably retransferred and stain the intended images. Mere touch in handling of the image-formed sheet often removes the images therefrom.
Furthermore, when the base sheet having a thermally transferred ink image is subjected to pressure-sensitive retransfer, the sheet is not easily fixed on an image-receiving substance since it has an extremely small coefficient of static friction, resulting in failure of image-retransfer getting out of position or with distortion.
In order to prevent the base sheet from slipping, it is described in JP-A-63-246298 to provide a sticky layer apart from thermally transferred images on the base sheet. However, an additional means is required to provide such a sticky layer at predetermined portions of the base sheet, and a device for the above purpose is needed. Further, the sticky layer has to be covered with a separable sheet, etc. before use, requiring further additional means and costs. In the case of using a silicone resin-coated sheets as a base sheet as described above, two layers, i.e., the silicone resin layer and the sticky layer must be provided and it may well be that one of the two layers previously coated has influence on the other. That is, when a silicone resin is first coated on a sheet, a sticky material is repelled when coated on the silicone resin layer. When the sticky material is first coated at portions of a sheet, its stickiness makes it difficult to coat the silicone resin on the sheet. Even if a separable sheet is provided on the sticky layer, difficulty in coating of the silicone resin cannot be eased because of increased thickness at the sticky layer-formed portions. Even with the two layers properly coated, fixation of the sheet is yet insufficient as the sticky layer exists only in portions not fully surrounding areas to which thermally transferred images are provided.
In order to solve the above problems, the present inventors previously made an intensive study and proposed an image-retransferable sheet having a transferable layer of surface treating agent, as described in U.S. Pat. application Ser. No. 408,236 filed Sep. 18, 1989, now abandoned. The surface treating agent used therein has a tensile strength of from 1 to 100 kg/cm.sup.2 and a melting or softening point of at least 100.degree. C. or a melt viscosity at 100.degree. C. of at least 1,000 poises. The present inventors also provided another image-retransferable sheet having a surface treating agent layer exhibiting a certain elongation, as described in U.S. Pat. application Ser. No. 405,279 filed Sep. 11, 1989, now U.S. Pat. No. 4,973,509. The surface treating agent contains an ethylene/vinyl acetate copolymer or polyethylene and exhibits an elongation of at least 100%.
In the case of pressure-sensitive retransfer of large images from the image-retransferable sheets, the opposite surface to the image-bearing surface of the sheet has to be rubbed thoroughly and patiently, generating unpleasant smell from the ink images or the sheet so that practitioners feel badly displeased and are annoyed with the smell during the operation of image-retransfer.